This invention relates generally to a method of and a device for continuous conditioning of sludges, particularly of sewage sludges, and more specifically it relates to a method in which the raw sludge is treated at an increased pressure and at an increased temperature while passing through successively arranged treatment stations until it is subject to a pressure required for the conditioning and subsequently the conditioned sludge is returned through the respective stations and is successively subject to decreasing pressure; at the same time the raw sludge, when passing through the consecutive stations, is heated up by vapors released during the depressurization of the conditioned sludge in respective stations and is thus preheated to a saturation temperature.
In thermal sludge conditioning it is more advantageous for regeneration of heat to use a direct heat exchange instead of an indirect heat exchange, because the direct heat exchange requires simpler equipment in the conditioning installation which is less susceptible to failure and also requires less maintenance. On the other hand, a sludge conditioner based on direct heat exchange is more expensive with regard to the measuring and controlling devices.
A method of the aforedescribed type is described in the German Pat. No. 20 19 731. In this prior-art method the pressure in respective stations is adjusted by means of exhaust gas releasing valves whereby it is necessary first to compute from a predetermined conditioning pressure in the reaction station the optimum pressures in respective heat exchanging stations and the gas releasing valves are adjusted according to the computed results so that when a computed pressure is exceeded a corresponding amount of exhaust gas is released.
This known control is possessed, however, of several disadvantages. If, for example, it is desired to change the pressure in the reactor for any reason, then it is necessary to change the pressures in all the remaining heat exchanging stations. In addition, this method does not make it possible to compensate for pressure deviations during which pressure in a station is below the computed optimum level. To correct such deviations, additional measures are necessary such as, for example, the intake of extraneous steam into the station in question. These equipment for effecting such measures represent additional investment expenditures and increased operational costs. Furthermore, the discharge of gas from the intermediate stations naturally causes heat losses and it is, therefore, desirable that such gas releases be limited to a minimum. The discharge of a certain amount of gas, however, is necessary because the atmosphere in individual stations which under ideal conditions should consist of pure steam only, is in effect formed of a gaseous mixture containing steam enriched with a smaller or larger amount of gaseous components, primarily carbon dioxide, which impairs the heat exchange.